Coextruded blown films are widely used in a variety of packaging as well as other applications. Film properties are often subject to the combined effect of the coextrusion process conditions and polymer compositions selected for the different layers. In order to address requirements of particular end-uses, film producers have to accordingly highlight certain film properties while balancing different mechanical properties such as stiffness and toughness to make stronger films for a given thickness and optical properties such as clarity and haze which impact the attractiveness of the packaging and visual inspection of the goods at the point of sale. A three-layer structure has been conventionally employed and refined over time in the art by different applications, in which at least one surface or outer layer is made to facilitate heat-sealing and a core layer may be used to provide strength, impact resistance, stretchability, other main physical properties of the film, or combinations thereof.
Among all polymers used for coextruded blown films, ethylene polymers, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) prepared by Ziegler-Natta catalyst in a gas phase process, and blends thereof, have been popular in the market because they can provide relatively low-cost solutions. Such materials, while offering advantages in clarity, often have negative effects on film mechanical properties for a number of applications. Efforts to remedy the situation include introducing and increasing metallocene polyethylenes (mPEs) in the films, which are also known to have extended versatilities and down-gauging potential of the three-layer coextruded blown films. However, addition of higher amounts of mPE into the three-layer films with reduced use of LDPE, while providing improved mechanical performance such as stiffness, may likewise lead to undesirable optical properties, such as a higher haze and a lower clarity. Generally, previous attempts in the art mostly focus on point-by-point improvements on the current three-layer structure, thus balance between different properties, flexibility in modification, and further potential of down-gauging all continue to be restricted by varying film formulation with the available selection of ethylene polymers. It is viewed as a difficulty by film manufacturers of various applications to develop a convenient and flexible approach to enable selective improvement on a certain set of properties per favored by end-use while maintaining a well-balanced overall film performance without significantly increasing polyethylene consumption under cost pressure.
WO 2014/088827 provides polyethylene compositions comprising one or more ethylene polymers and one or more HDPE modifiers, in particular, this application further relates to polyethylene blends comprising one or more ethylene polymers and one or more HDPE modifiers, wherein the modifier has: 1) a density of greater than 0.94 g/cc; 2) a Mw/Mn greater than 5; 3) a melt index (ASTM 1238, 190° C., 2.16 kg) of less than 0.7 dg/min; and 4) a g+vis of 0.96 or less.
U.S. Patent Publication No. 2012/0100356 relates to a multi-layer blown film with improved strength or toughness comprising a layer comprising a metallocene polyethylene (mPE) having a high melt index ratio (MIR), a layer comprising an mPE having a low MIR, and a layer comprising a HDPE, and/or LDPE. Other embodiments have skin layers and a plurality of sub-layers. At least one sub-layer includes an mPE, and at least one additional sub-layer includes HDPE and/or LDPE. The mPE has a density from about 0.910 to about 0.945 g/cm3, MI from about 0.1 to about 15 g/10 min, and melt index ratio (MIR) from about 15 to 25 (low-MIR mPE) and/or from greater than 25 to about 80 (high-MIR mPE). The process is related to supplying respective melt streams for coextrusion at a multi-layer die to form a blown film having the inner and outer skin layers and a plurality of sub-layers, wherein the skin layers and at least one of the sub-layers comprise mPE and at least one of the sub-layers comprise HDPE, LDPE or both. Draw-down, blow-up ratios and freeze-line distance from the die are controlled to facilitate a high production rate.
U.S. Pat. No. 8,586,676 provides a polymer composition and articles made therefrom. The composition includes: (a) a polyethylene having (i) at least 50 wt % ethylene moieties; and (ii) up to 50 wt % of a C3 to C20 comonomer moieties, a density of about 0.860 to about 0.965 g/cm3, a melt index of about 0.1 to about 10.0 g/10 min and a branching index of about 0.96 to about 1.0; and (b) a polyethylene having: (i) at least 65 wt % ethylene moieties; and (ii) up to 35 wt % of a C3 to C20 comonomer moieties, wherein the wt % is based upon the total weight of the latter polyethylene, a density of about 0.905 to about 0.945 g/cm3, a melt index (MI) of about 0.1 to about 10.0 g/10 min, and a branching index (g′) of about 0.7 to about 0.95.
WO 2009/109367 discloses the use linear polyethylene having an MIR indicative of the presence of some long chain branching having a density of 0.91 to 0.94 g/cm3 determined according to ASTM D4703/D1505, an I2.16 (MI) of from 0.05 to 1 g/10 min, and I21.6/I2.16 (MIR) of more than 35, the MI and MIR being determined according to ASTM 1238 D at 190° C., and a difference between the MD Tensile force based on ASTM D882-02 at 100% elongation and MD 10% offset yield of a reference film as defined herein having a thickness of 25 μm of at least 15 MPa. This patent application also relates to coextruded film structures made using such linear polyethylene in the core layer of a multi-layer structure to provide easily processable, strong, highly transparent films.
U.S. Pat. No. 6,956,088 relates to films that exhibit an improved balance of physical properties, and a metallocene catalyzed polyethylene used to make the films that is easier to process than previous metallocene catalyst produced polyolefins and/or polyethylenes. The films are produced with polyethylenes having a relatively broad composition distribution (CD) and a relatively broad molecular weight distribution (MWD).
That said, there remains an industry wide need to explore a solution that can deliver advantages over the current three-layer structure technology of expanded selection of options to more efficiently improve application-oriented properties without substantially compromising other properties, preferably with gauge reduction. Applicant has found that such objective can be achieved by a film structure of at least five layers as long as certain density relationships between layers are met. While the polyethylenes used remain unchanged, compared to the conventional three-layer structure, such increase in the number of layers can facilitate selective improvement on desired properties and fine-tuning of property profile by conveniently adjusting layer distribution and density relationship between layers, particularly modifying relationship between other layers and the most “stiff” layer, i.e. the one containing the polyethylene having the highest density of all polyethylenes in the film, preferably the inner layer between the core layer and the outer layer. In step with the above is an improved balance between repulsive mechanical properties, e.g., stiffness-related and toughness related properties, as well as between mechanical and optical properties, which results in enhanced overall film performance allowing for a gauge reduction of at least about 10%, depending on specific property profile. Therefore, the inventive film offers a promising alternative to the conventional three-layer structure for future development in the coextruded blown film industry.